It is a manually controlled closed circuit rebreather with back mounted
split counterlungs, axial flow scrubber, and three independent P02 (partial
pressure of oxygen) displays.
Oxygen Control
The
unique feature of the oxygen system is the addition of an oxygen metering
orifice. This is NOT a constant mass flow device like the ones used in
semi closed rebreathers. What it does is add a small flow of oxygen that
is normally set at a surface rate of .7 - 1.0 liters per minute. This
addition of 02 greatly reduces the amount of time the diver has to spend
adding oxygen. It is in no way a controller and does not reduce the need
for the diver to constantly monitor the oxygen level in the breathing
loop. As the depth increases the oxygen flow rate decreases due to the
increasing ambient pressure and at the same time the rate of change of
the P02 slows due to the increased gas density. As much as I would like
to claim that these two complimentary features are a result of brilliant
design engineering they are in fact just basic physics.
 All
closed circuit rebreathers require the diver to monitor the P02 regardless
of how many computers are built into the rebreather. The fact that the
KISS rebreather also requires you to push the button on the oxygen add
valve once in a while generally hasn't been seen as a major inconvenience.
All CCR training courses teach the student how to manually control the
PO2 before allowing them to turn on the computer. It isn't difficult to
do. In fact many divers with computer controlled rebreathers have elected
to disable the computer and operate the rebreather in manual mode at all
times. If you are diving in a task loaded situation such as doing photography,
video, spearfishing, or defusing explosive mines then a fully automatic
computer controlled rebreather may be your best choice.
P02 Displays
The
KISS rebreather has three independent P02 displays. Each of these displays
is enclosed in it's own housing, with it's own battery, and reads it's
own separate sensor. The probability of all three of these displays failing
simultaneously is rather remote. Each display can be replaced independently
and spares are easily affordable.
Scrubber
The scrubber is a basic axial flow "there and back" design.
It holds approximately 2.7Kg of 4-8 Sofnolime. The axial flow design is
resistant to "channeling" (allowing the gas to bypass the scrubber
bed) but has higher breathing resistance than radial flow designs. Everything
is a compromise. It is rated for three hours in cold water. Some people
push them to 4-5 hours and some have done some long surface swims as a
result. A C02 hit underwater is an unbelievably ugly experience that can
easily prove fatal. This is not an area to be cheap, change it often and
start every deep dive with a fresh fill of Sofnolime.
Counterlungs
The rebreather uses two back mounted counterlungs (split counterlung)
that are avaliable in three sizes; 2, 4, and 6 liter capacity. Some combination
of these will normally allow a good match between the counterlung volume
and the divers lung capacity. This match permits easier control of buoyancy.
Back mounted counterlungs leave the chest area clear and reduce the number
of hoses and fittings compared to over the shoulder counterlungs found
on other rebreather designs. They are also subject to changes in breathing
resistance as the diver changes positions in the water. If you roll on
your back you can expect a case of chipmunk cheeks. Nothing is perfect.
DSV (Mouthpiece)
 I
used to consider a mouthpiece that could be switched between rebreather
and open circuit (OC) mode an essential feature on a rebreather. I don't
consider it essential anymore but it is still nice to have. Being able
to go to open circuit with just a twist of a knob is convenient for many
reasons. It can provide an alternate method of adding diluent gas or even
an alternative diluent depending on the connection. It is an easy way
to purge the rebreather for verifying the sensor readings and it can prevent
panic in the case of a malfunction in the rebreather by allowing a fast
simple way of getting your next breath. The drawback of the built in bailout
mouthpiece is the size and the fact that the OC regulator has to be de
tuned to keep it from freeflowing.
ADV (automatic diluent valve)
This is another feature that I initially considered essential but is
actually in the "nice to have" category. This valve adds diluent
when the loop volume is reduced by either descending or "breathing
down" the volume of oxygen in the loop. Suddenly finding yourself
unable to get a breath during a rapid descent could be a panic inducing
situation but it is obviously something that a diver can get used to since
Buddy Inspiration divers do it all the time. The ADV needs to be set up
"tight" enough that it doesn't add diluent without the diver
being aware of it but it needs to add enough gas so that a reasonable
descent rate can be maintained. Any time the ADV triggers you need to
check your P02 because you have either descended and compressed the gas
in the loop or you have consumed enough oxygen to reduce the P02 significantly
which may also have caused you to loose buoyancy and descend.
Tanks
I
recommend using 13 cubic foot (2 liter) tanks. The limiting factor on
the rebreather is the 3 hour scrubber. A 13 cubic foot oxygen tank will
provide 5 hours of oxygen at a consumption rate of 1 liter per minute.
A 13 cubic foot diluent tank will provide enough gas for an experienced
rebreather diver to do two hour long dives to 150 feet. If you carry more
gas than this, and you should, it should be in the form of emergency bailout
open circuit gas in separate tanks. A larger diluent tank is not an adequate
bailout gas supply. Carefully analyze the failure paths on your gas supply
and don't put all your eggs (gas) in one basket (tank).
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